Method of lessening the inhibitory effects to fluid flow due to the presence of solid organic substances in a subterranean formation



METHOD OF LESSENING THE INHIBITORY EFFECTS TO FLUID FLOW DUE TO THE PRES- ENCE OF SOLID ORGANIC SUBSTANCES IN A SUBTERRANEAN FORMATION Curtis Wendell Crowe, Tulsa, Okla., assignor to The Dow Chemical Company, Midland, Micl1., a corporation of Delaware No Drawing. Filed Apr. 6, 1967, Ser. No. 628,850

Int. Cl. E21b 43/00, 43/25, 43/119 US. Cl. 166307 11 Claims ABSTRACT OF THE DISCLOSURE Organic substances including bacterial slimes, algae, synthetic polymers and the like, in the vicinity of a wellbore penetrating and geologic formation, and rendered less inhibitory to passage of fluids to the wellbore by contacting such organic substance with an aqueous composition comprising a sodium hypohalite containing an inhibitor to the corrosivity thereof to metal of which water-dispersible or water-soluble silicates and hydroxides are particularly effective. Most effective results are obtained when the sodium hypohalite-inhibitor solution is followed by an acid flush.

The invention provides a long experienced need in the art of stimulating flow of fluids from a substerranean reservoir by a wellbore wherein such flow is impaired by the presence of organic substances of which bacterial slimes, algae, and polymeric substances are among the more common interferents. Reference to a geologic formation so impaired will sometimes be made herein as an organiccongested formation.

The invention is applicable to the treatment of any porous formation. It is particularly useful in treating water-sensitive, fluid-bearing sandstone strata. Such sandstones almost invariably contain some striations or eroded particulate material that renders them Water-sensitive to some extent.

The polymeric substances which are most troublesome are frequently polymers or partially degenerated polymers that had previously been injected down the well to attain a specific objective associated with a previous formation treatment. For example, various polymers are useful to lessen fluid-loss of slurries and solutions injected down a wellbore and into a formation or at least in contact therewith including hydraulic fracturing and acidizing compositions. Polymers are also useful as diverting agents in formation treatments. Sometimes an appreciable proportion of the polymer used remains in the interstices of the formation, particularly in the vicinity of the wellbore, often undergoing bacterial action and partial degradation which renders it particularly obstructable to fluid flow.

Other polymeric substances that are troublesome are naturally occurring substances of both small animal and plant life including algae which sometimes grow and multiply around a wellbore to result in a state of near of substantial impermeability.

Treating agents amounting to a rather imposing list have been considered and dried in efforts to lessen these accompanying undesirable effects of the presence of such organic substances. None heretofore has proved fully satisfactory.

Among the more promising materials have been certain chlorine-containing substances which lessen bacterial action, tend to degrade any polymers present, and in general lessen th impairment of permeability properties of the affected formation. The better bactericide employed has been sodium hypochlorite. However, its corrosive attack on metal parts, e.g. pipes, pumps, valves, and the ited States Patent O like has been extremely severe. Efforts to inhibit the corrosivity thereof on metal have not been fruitful. No inhibitors heretofore known to be useful in acidic media, e.g. HCl, H and the like, have performed in as effective a manner as was hoped.

The invention may be summarized as a method of effectively destroying organic deposits and accumulations in wells, and formations penetrated thereby, without objectionable detrimental effect on the metal parts encountered or contacted thereby, which encompasses injecting down the well, and back into the organic-congested formation, a composition comprising a hypohalite, selected from the class consisting of water-soluble hypochlorites and hypobromites, and an alkali metal hydroxide inhibitor to the corrosivity of the hypohalite to metals, in an amount sufficient to give an alkaline pH value of at least about 13.

The hypohalites include sodium and potassium hypochlorites and hypobromites and mixtures thereof. The hydroxides include NaOH, KOH, and LiOH. The silicates include all generally known fused SiO and Na O mixtures having a ratio of SiO /Na O of from about 0.5 to about 4. This range includes sodium ortho-silicates (a molar ratio of SiO /Na O of 0.5); meta-silicates (a molar ratio of SiO /Na O of 1); and more siliceous sodium silicates, e.g.

* SiO /Na O of 3.22. The silicates often exist as the hydrates.

Silica gels (gelatinous highly hydrated sodium silicates) and water glass (aqueous solution of meta-silicates) may be used in the practice of the invention.

For illustrative purposes, sodium hypochlorite, sodium hydroxide, and/ or Na SiO '9H O are selected below and the following operational and preferred amounts thereof are recommended for treatment of a geologic formation congested by organic substances including natural and synthetic polymers, at various stages of growth or degeneration and decay.

Consentration Limits Operative Preferred Sodium hypochlorite 0.1%20% 1. 0%10% Sodium hydroxide T 0. 5%40% 2. 0%10% NilzSlOg-QHgO (Sodium Silicate) 2%-40% 3%10% The invention is practiced by injecting, as by pumping, an aqueous solution of the hypohalite, e.g. sodium hypochlorite, and the inhibitor, e.g. sodium hydroxide Series one A high molecular weight copolymer was prepared by copolymerizing a monomeric mixture consisting, by weight, of about 40% of N-vinyl-2-pyrrolidone and 60% of acrylamide, cross-linked with 0.7% by weight N,N- methylenebisacrylamide in the presence of a free radical promoting catalyst, substantially as described in patent application Ser. No. 482,621, filed Aug. 25, 1965. For

brevity, such polymer is usually referred to hereinafter as Polymer X. One gram of the copolymer so made was dissolved in successive milliliters volumes of various selected liquids at F. and atmospheric pressure. The tests were for the purpose of showing the extent to which the polymer dissolved in the various selected liquids. The liquids employed and the extent of the resulting dis- Series three This series of tests was conducted to show the inhibiting properties of sodium silicate on the corrosivity of sodium hypochlorite in contact with steel. The silicate employed was Na SiO -9H O. The aqueous solution again consisted of a 5% by weight solution of sodium hypo- TABLE I.EFFECT OF VARIOUS REDUCING AGENTS ON SWOLLEN [Test Temperature 175 F.; One Gram of Dry Polymer Added to 100 ml. of Liquid] Test Identification Solvent Results 1 Water only No change in 24 hrs.

2. Water containing 1% sodium persultate Slight degradation in 24 hrs.

3 Water containing 5% sodium persuliate Moderate degradation in 24 hrs. 4 Water containing sodium persulfate 50% degradation in 24 hrs.

5 Water containing sodium persulafte Total degradation in 3 hrs.

6 Water containing 1% potassium periodate Do.

7 Water containing 5% hydrazine No ch nge in 24 hrs.

8 Water containing 10% hydrazine Do.

9 Water containing 17% hydrazine Do.

10 Water contianing 1% hydrazine sulfate Do.

11 Water containing 5% calcium hypochlorite Rapid degradation but considerable residue remained at both concentrations.

12 Water containing 10% calcium hypochlorite. Do.

13 Water containing 3% hydrogen peroxide No change in 24 hrs.

14 Water containing 5% sodium hypochlorite Total degradation within 10 min.

Reference to Table I shows that only sodium hypochlorite, Test 1, illustrative of the practice of the invention was a truly effective solvent for the polymer. Calcium hypochlorite (used in comparative Tests 11 and 12), attacked the polymer but formed large volumes of calcium hydroxide which make its use impractical in earthen formations. Hydrogen peroxide (Test 13), hydrazine (Tests 7, 8 and 9), and hydrazine sulfate (Test 10), for example, were shown to be completely ineffective for dissolution of the polymer.

Series two This series of tests was run to show the corrosion rate on steel pieces in contact with aqueous sodium hypochloride solutions. Steel coupons identified in the metal industry as N80 steel (a common carbon steel) were placed in suitable containers and to each container was added a solution of sodium hypochlorite containing either none or various percents of NaOH as an inhibitor to the corrosivity of the hypochlorite on the steel. Corrosion rate was ascertained by weighing the steel coupons of known total area both before immersion in the solution and after removal therefrom and drying and calculating the weight loss per square foot of metal surface.

TABLE II.CORROSION OF 5% SOLUTIONS OF NaOCl, CON- TAINING NaOH, UPON N80 TUBING [Pressure: Atmospheric; Temperature: 175 F.; Time: 24 Hours] Corrosion rate of metal in pounds/ square foot NaOH concentration in gm./l00 ml. of solution Test1 Identification:

ozoooqammonmo chlorite containing from 0 to 9% by weight of the silicate. The temperature was 175 F.; the time was 24 hrs.; the pressure was atmospheric. The results are set forth in Table III wherein increasing amounts of the sodium silicate hydrate were admixed with successive samples of the hypochlorite; the corrosion rate of each determined.

TABLE TIL-CORROSION OF 5% SOLUTION OF NaOCl, CONTAINING Na SiO .9H O, UPON N TUBING Pressure: Atmospheric; Temperature: 175 F.; Time: 24 Hours] Corrosion rate in pounds of metal/ square foot N a SiOIQH O concentration in gms./ ml.

sco qcaenwwrowo Reference to Table III shows that sodium silicate is an effective inhibitor to the corrosivity of the aqueous hypochlorite solution on steel. Although the inhibitory effect is not as pronounced as when the NaOH was used, it is nevertheless highly effective when used in amounts of at least 2 grams of the sodium silicate hydrate per 100 milliliters of the hypochlorite solution.

Series four This series of tests was conducted to show the effect, if any, of the NaOH on the stability of the sodium hypochlorite solution. The pressure, temperature, and time observed were the same as in series three. N80 steel coupons were again employed in the successive solutions of the hypochlorite. The amounts of NaOH present and the results obtained are set out in Table IV.

TABLE IV.STABILITY OF 5% SOLUTIONS OF NaOCl,

CONTAINING NaOH, IN CONTACT WITH STEEL [100 ml. of solution in contact with one standard N80 coupon] Pressure: Atmospheric; Temperature: F.; Time: 24 hours.

Reference to Table IV shows that the NaOH is a valuable aid in preventing decomposition of the sodium hypochlorite. It acts to preserve or improve the stability thereof and accordingly serves a two-fold purpose in the practice of the invention.

Series five The effect upon the dissolution rate of a polymer by the sodium hypochlorite solution was shown in this series of tests. It was shown in series one that the sodium hypochlorite solution was highly effective to dissolve the difficultly soluble polymer there employed. The same polymer was employed in this series of tests but the sodium hypochlorite solution (with the exception of Test 47) contained increasing amounts of NaOH for the purpose of ascertaining its effect, if any, on the rate of dissolution of the polymer.

TABLE V.-EFFECT OF NaOH CONCENTRATION IN 5% NaOCl UPON DISSOLUTION RATE OF POLYMER X [2 gm. of :80 polymenwater in gel state added to 100 ml. of liquid] Test Temperature: 175 F.

NaOH concentration in gm. Dissolution 100 ml. of time in solution minutes H ocooosrmcnuawtovc Reference to Table V shows that, although the presence of NaOH in the sodium hypochlorite solution lessens the rate of dissolution of the polymer, the solution nevertheless remains highly effective as a solvent for the polymer and appears to be but little changed by the amount of the NaOH present.

Series six TABLE VI.-EFFECT OF TEMPERATURE ON DISSOLUTION RATE OF POLYMER IN 5% NaOCl AND 6% NaOH SOLUTION [2 gms. of 20:80 polymerzwater in gel state to 100 ml. of liquid] Dissolution Temperature time in in F. minutes Reference to Table VI shows that the time required to dissolve the polymer decreases with the rise in temperature. In other words, the rate of dissolution is inverse to the temperature. However, excellent results are obtained at any temperature of 150 F. or above and acceptable results may be considered obtainable at as low as 100 F. Best results would be obtained in formations having temperature of 175 F. and higher.

Series seven This series of tests, numbered 62 to 70 was conducted to show the effect of a composition required by the practice of the invention, as illustrated by a 5% NaOCl-6% NaOH aqueous solution, on Berea sandstone cores, including those damaged by Water, those undamaged by water, and those impregated with natural or synthetic polymers. The tests were conducted by preparing two-inch diameter cores of suitable length and placing the cores in a Hassler-type permeability cell as described in API RP 40 First Ed. (August 1960) page 35, and depicted there as FIG. 3.5.15F2. The various permeability values were then obtained on the various cores after passing each of a series of aqueous solutions therethrough, as described below, specifically the results thereof being set out in Table VII, infra.

Test 62.--A Berea core was tested by passing a 3% by weight aqueous solution of NaCl (brine) therethrough, determining the permeability, then passing fresh water therethrough and again determining the permeability.

Test 63.A second Berea core was tested by passing first a 3% aqueous solution of NaCl, then a 5% NaOCl-6% NaOH solution then additional 3% NaCl solution, and finally fresh water. The effect on permeability by each liquid was determined.

Test 64.A third Berea core was tested by first passing API brine (2% CaCl and 8% NaCl by weight aqueous solution) therethrough, then 5% NaOCl-6% NaOH solution, then more API brine. The effect on permeability by each liquid in sequence was determined.

Test 65.A fourth Berea core was tested by first passing therethrough 3% NaCl aqueous solution, then 5% NaOCl-6% NaOH solution, thereafter fresh water, and again 5% NaOCl-6% NaOH. The effect on permeability by each liquid was determined in sequence.

Test 66.-A fifth Berea core was tested by first passing therethrough a 3% aqueous solution of NaCl followed by an aqueous solution of an inhibited 15% HCl solution. The effect on permeability values was determined by each liquid.

Test 67 .-A sixth Berea core was tested by first passing therethrough 3% aqueous NaCl, thereafter impregnating the core with an alginate slime scraped from water coolers and dispersed in 3% NaCl brine; then passing a 5% NaOCl-6% NaOH solution through the core, and finally passing 5% by weight aqueous uninhibited HCl solution therethrough. The effect on permeability of each liquid was determined in sequence.

Test 68.A seventh Berea core was tested by first passing a 3% NaCl solution therethrough, then impregnating the core with alginate slime scraped from water coolers and dispersed in 3% NaCl brine thereafter a 15 aqueous solution of inhibited HCl, and finally a 5% NaOCl- 6% NaOH solution. The effect on permeability by each liquid was determined.

Test 69.-An eighth Be'rea core was tested by passing first a 3% aqueous solution of NaCl therethrough, then impregnating the core by passing a dense slurry of Polymer X dispersed in 3% NaCl brine thereinto, thereafter passing a solution of 5% NaOCl-6% NaOH therethrough, and finally passing a 15% aqueous solution of inhibited HCl therethrough. The effect on permeability by each liquid was determined.

Test 70.A ninth Berea core was tested by passing first a 3% aqueous solution of NaCl therethrough, then impregnating the core by passing a dense slurry of Polymer X in 3% aqueous NaCl therethrough, thereafter passing a solution of 5% NaOCl-6% NaOH, and finally a 15 aqueous solution of inhibited HCl therethrough. The effect on permeability by each liquid was dete mined.

By inhibited HCl acid or inhibited HCl aqueous solution is meant such acid or solution containing an effective amount, e.g. 0.05 to 1.0% by weight of an inhibitor to acid corrosive attack on metals. The inhibited acid used in this series of tests was 15% aqueous HCl con aining 0.4% by weight of the inhibitor described in US. Patent 3,077,454.

7 8 The various permeability values in millidarcies are set Miocene formation at a depth of 7,880 feet. Bottom hole out in Table VII wherein each permeability value is temperature was 175 F. The well had been provided entered on the table directly below the description of the with a 16-foot gravel pack (comprising 20 40 mesh sand) aqueous liquid which was injected into the core. at the producing interval at the level of 7,864 to 7,880

TABLE VII.PERMEABILITY CHANGES RESULTING FROMVARIOUS CORE TREATMENTS STATED IN MILLIDARCIES (1nd.)

Berea Sandstone Cores Tested at 500 p.s.i. Injection Pressure at 125 F. (Except Test 70 at 175 F.)

Teal;

Identification Each Core Pie-saturated with 3% N aCl Brine, as Shown 62 3% NaCl /l4. O md. Fresh Water/0.8 1nd 63. 5% NaOCl-6% NaH /8.0 md 3% NaCl/ll 0 ind... Fresh Water] 1.8 1nd.

66- 3% NaC1/29 0 md Inhibited 15% HUD/25 0 md 67- 3% NaGl/22.6 md Slime in 3% NaCI /ZJ Ind- 5% NaOCl-6% NaOH/6.8 HC1/17.1 md.

68- 3% NaCl/31 md Slime in 3% NaCl/1.6 Ind Inhibited 15% HOl/l.0 md 5% NaOCl-6% NaOH/3.4md. 69 3% NaCl/29.5 md Polymer X in 3% NaOl /1.8 md 5% Na0Cl-6% Na OH/1.5 md- Inhibited 15% HCl/4.5 md. 70 3% NaCl/23.3 1116.... Polymer X in 3% NaCl Solution/1.2 md- 5% NaOOl-6% NaOH/0.8 md. Inhibited 15% HCl/33.5 md.

1 All refer to the material dissolved in enough water to make 100% by weight. Reference to Table VII clearly supports the following feet to prevent sanding in of the Well. The well had conclusions, based on the tests of various Berea cores, been acidized employing a dispersed polymer in the each pro-saturated with a 3% NaCl brine: aqueous acidizing solution and following treatment was Test 62.The introduction of fresh water into the core not producing as expected. It appeared that an excessive caused a marked decrease in permeability, actually the amount of polymer had been used for the conditions expermeability was somewhat less than 6% of the permeaisting in the formation and such excess had not been rebility to a 3% NaCl brine. turned by recirculating as is usual under such circum- Test 63.-The introduction of the sodium hypochloritestances. The Well was equipped with a casing and tubing sodium hydroxide solution (the treating composition acand a packer positioned in the annulus between casing cording to the invention), followed by NaCl brine and d tubing above the producing interval. then fresh water showed that such solution lessened the Th ll was t t d according t th invention, as adverse eifect of fresh water on the permeability. f 11 Test 64.-The introduction of the treating composition Thg hf between tubing d casing wa filled of the invention into the core had no significant effect upon i h b i Th bi was th fill d i h di l f l the core permeability to 3% NaCl brine, i.e. a formation and a pump i rate, i h f ti f -h 1f is not damagfid y the Composition P y in the barrel per minute at 600 pounds per square inch pres- V n sure was established. This was followed by 1,000 gallons T -Th introduction of the treating p of a solution composed of 5% sodium hypochlorite and tion employed in the invention cannot correct the damage 5% Sodium h d id Thi l i was pumped i Previously done by fresh Waterincrements of 100 gallons with a 30 minute period T lreatment of a test Core by inhibited allowed between each 100 gallons pumped. Consequently, HCl acid does not adequately improve the permeability; 5 hours were required to pump the entire 1 000 ll s a reduction in Permeability resulted of the 5% sodium hypochlorite-5% sodium hydroxide T est 67-WheI1 dispersed bacterial Slime is forced into solution. This solution was followed by 200 gallons of a the NaCl brine saturated core until the permeability is re- Water spawn The Spacer was f ll d i 2 000 l- (1116661 to and thereafter the NaOCI'NaOH lons of 15% hydrochloric acid containing 0.2% hexynol. ployed in the invention passes therein, a very gratifying in- The hexynol was present as an inhibitor to prevent i- C a in Permeability ensued: approaching that of the cal attack of the acid upon the metal. A sufficient amount Original of diesel oil was then pumped to force the acid remain- TeSi a Core is treated Similarly to Test ing in the tubing into the formation and leave the tubing x p that the Order of u$i11g I iaOC1'NaOH i the Hci full of diesel oil. The well was then shut in and allowed acid is reversed, the permeability of the core is shown to to stand for 24 hours be badly impair i Prior to the treatment, the well had been producing at Test 69.Wh a Core 15 lmpliegnated with Polymer a rate of only 18 barrels of oil per day. Following the X cl'ossihnked 60% acrylamlde and 40% N'viiiyi' treatment, production was 140 barrels of oil per day,

pyrrolidone copolymer) and subsequently treated suca striking and gratifying improvement cessiveiy with the NaOCi'NaOH solution used. in i The above described field example illustrates the pracveiiiioii inhibited 15% the Peiimeabiiiiy is i tice of the invention, but includes certain steps includ- Proved to some extent but not to the exieiii of the ing filling the annulus with a brine and then injecting movement attained against the natural Siime' down the tubing a pre-acid treatment prior to the injec- Tesi 70 HOWever when the temperature of the i tion of the inhibited polymer-solvent according to the is raised 50 Faieiiheii degiees (to 175 Very sails invention. The after flush acid treatment employed in factory Permeability is regained iii a formation Piugged this example illustrates a preferred practice of the invenby the dithc fltly degraded polymer, by use of the NaOCltion as aforesaid N H SOIHUOH- Only the injection of the aqueous solution of the by the diiiici'iiiiy degraded polymer by use of the selected water-soluble hypohalite containing the water- NaOC1 NaOH soluble silicate or hydroxide is essential. The after flush employing an aqueous acid solution illustrates the preferred embodiment. The other steps observed in the field example are non-essential for the practice of the invention.

Having described my invention, what I claim and desire to protect by Letters Patent is:

solution.

The tests of series seven show the effioacy of the sodium hypohalite-sodium hydroxide aqueous solution, in general, on cores taken from water-sensitive geologic formations. The tests show especially the efficacy 0f the practice of the invention when the hypochlorite-hydroxide 1. The method of improving the permeability of a solution is followed by an acid flush, sometimes called an Porous geologic formation wherein Organic polymeric acid Chaser' 1d am 1 materials are lodged in the pores which comprises injectle ex p 6 ing into the formation an aqueous composition compris- Test 71.An oil well in the South Pass, Block 24 Field ing water, a Water-soluble hypohalite, and a sufficient in Plaquernine Parish, Louisiana was completed in a '15 amount of an alkali metal hydroxide to result in an alkaline pH value of said composition of at least about 13 to inhibit the corrosivity, of the hypohalite to metal parts encountered, whereby said polymeric materials are at least partially solubilized to facilitate removal from the formation and the permeability thereof thus improved.

2. The method according to claim 1 wherein the hypohalite is sodium hypochlorite.

3. The method according to claim 1 wherein the hypohalite is employed in an amount of between about 0.1% and about 20.0%, based on the weight of the aqueous composition.

4. The method according to claim 3 wherein the amount of the hypohalite is between about 1.0% and about 10.0%, based on the weight of the aqueous solution.

5. The method according to claim 1 wherein the injection of the aqueous solution of hypohalite and inhibitor to the corrosivity thereof is followed in next succession by the injection of an aqueous acid.

6. The method according to claim 5 wherein the acid is a 2% to 30% aqueous solution of HCl containing an inhibitor to corrosivity of acid to metal.

7. The method according to claim 6 wherein the inhibitor to corrosivity of acid is that prepared in accordance with Example 2 of US. Patent 3,077,454.

8. The method according to claim 1 wherein the inhibitor to corrosivity of the hypohalite is between about 0.5 percent and about 10 percent, based on the weight of said aqueous solution.

References (Jited UNITED STATES PATENTS 1,834,783 12/1931 Johnson 252387 X 2,105,839 1/1938 McNutt 252-387 X 2,429,593 10/1947 Case.

2,560,331 7/1951 Buchan 1661 3,077,454 2/1963 Monroe et al.

3,122,503 2/1964 Katzer 16644 X 3,249,536 5/1966 Jones.

3,372,748 3/1968 Cook 116-9 OTHER REFERENCES Am. Pet. Inst, Secondary Recovery of Oil in the United States. New York, A.P.l. 2d. ed., 1950, pp. 363-364.

CHARLES E. OCONNELL, Primary Examiner US. Cl. X.R. 

